This invention relates to a medical procedure for correcting astigmatism by the execution of corneal incisions, and to marking devices for use in carrying out such procedures.
For a clearer understanding of the invention, a number of terms, in double quotes, will be defined, and it is understood that when those terms are used hereinafter in the specification or the claims they will have the meanings as herein defined.
A normal cornea is spherical and positions are located by equal meridians analogous to great circles on the surface of a globe. An astigmatic cornea is shaped similar to a USA football and has unequal meridians. The "steepest" meridian is the steepest or most curved meridian of the astigmatic cornea. The "secondary" meridian is the flattest and least curved meridian that is always transverse to or rotated 90.degree. with respect to the steepest meridian. For convenience of explanation, unless otherwise indicated, the steepest meridian will be considered to extend in the North (N) to South (S) direction and the secondary meridian in the West (W) to East (E) direction. The "bisector" meridians are the meridians which divide in two equal parts the quadrants between the steepest and secondary meridians, of which two exist: NW to SE and SW to NE. The "surgical" meridian is the meridian which passes through the center of transverse or arcuate incisions in the cornea and should always coincide with the steepest meridian unless it is executed improperly and thus extends along a line offset from the steepest meridian. The "axis of a meridian" is the straight line extending through the end points of the meridian.
When a circular light is projected over an astigmatic cornea, an oval reflection is produced called an "oval keratometric mire". When it is horizontal like the eye opening, the astigmatism is "with the rule"; when vertical, it is "against the rule". The "optical zone", is the area of the cornea through which light rays can pass through the pupil to the retina, and the "optical axis" is the axis passing through the center of the optical zone. The optical zone is measured in millimeters (mm) of its diameter and is typically about 5 mm in an adult. "Markers" are surgical instruments used by the surgeon to mark positions on the patient's cornea to assist the surgeon in locating the precise lines where incisions are to be performed. Typically, markers comprise a handle having at the distal working end an angled circular structure with projecting thin edges--also sometimes with crosshairs for aiming--which are pressed against the cornea after proper positioning to leave a visible mark or marks that the surgeon can use to locate the incisions. They are typically sold in sets wherein the working ends are scaled in size for use with patients with different degrees of astigmatism.
To correct astigmatism, which means to transform an astigmatic cornea into as nearly a spherical one as possible, various surgical techniques have been developed which are characterized by the execution of relaxing surgical incisions on opposite sides of the optical axis across the steepest meridian, the most curved part of the cornea. The underlying concept is that the incisions across the steepest meridian will cause the cornea to flatten along that meridian, thus tending to bring the curvature of the meridians closer together, hence shape the cornea more spherically. Most recommended surgical procedures have formed straight lines running parallel to the steepest meridian, or transverse to the steepest meridian, or both. One procedure recommended a pair of arcuate (segments of an arc) incisions from 45.degree.-90.degree. across the steepest meridian on opposite sides of the optical axis. These known surgical techniques have suffered from various drawbacks in that they not only have been less than fully successful in correcting many forms of astigmatism, but also have on occassion introduced new distortions in the patient's vision.